Fuel cells are power generating systems that convert energy produced through an electrochemical reaction of fuel and oxidative gas directly into electric energy. Such fuel cells can be categorized into electrolyte fuel cells containing molten carbonate salt, which are operable at a temperature between 500° C.–700° C., electrolyte fuel cells containing phosphoric acid, which are operable around 200° C., and alkaline electrolyte fuel cells and polymer electrolyte fuel cells, which are operable between room temperature and 100° C.
The polymer electrolyte fuel cells include proton exchange membrane fuel cells (PEMFCs) which use hydrogen gas as the fuel source and direct methanol fuel cells (DMFCs) which use liquid methanol directly applied to an anode as the fuel source. The polymer electrolyte fuel cells, which are emerging as a next generation clean energy source alternative to fossil fuels, have high power density and high energy conversion efficiency. In addition, the polymer electrolyte fuel cells function at an ambient temperature and are easy to hermetically seal and miniaturize. Therefore, they can be extensively applied to zero emission vehicles, power generating systems for home use, mobile telecommunications equipment, medical equipment, military equipment, space equipment, and the like.
The basic structure of a PEMFC as a power generator producing a direct current through the electrochemical reaction of hydrogen and oxygen is shown in FIG. 1. Referring to FIG. 1, the PEMFC includes a proton-exchange membrane 11 interposed between an anode and a cathode.
The proton-exchange membrane 11 is composed of a solid polymer electrolyte with a thickness of 50–200 μm. The anode and cathode respectively include anode and cathode backing layers 14 and 15 for supplying reaction gases, and catalyst layers 12 and 13, in which the oxidation/reduction of reaction gases occurs, thereby forming the gas diffusion electrodes (hereinafter, the anode and cathode will be referred to as “gas diffusion electrodes”). In FIG. 1, a carbon sheet 16 has gas injection holes and acts as a current collector. Hydrogen, as a reactant gas, is supplied to the PEMFC, and hydrogen molecules decompose into protons and electrons through an oxidation reaction in the anode. These protons reach the cathode via the proton-exchange membrane 11.
Meanwhile, in the cathode, oxygen molecules receive the electrons from the anode and are reduced to oxygen ions. These oxygen ions react with the protons from the anode to produce water. As shown in FIG. 1, within the gas diffusion electrodes of the PEMFC, the catalyst layers 12 and 13 are formed on the anode and cathode backing layers 14 and 15, respectively. The anode and cathode backing layers 14 and 15 are composed of carbon cloth or carbon paper. The surfaces of the anode and cathode backing layers 14 and 15 are treated so that reaction gases and water can easily permeate into the proton-exchange membrane 11 before and after reaction.
Although a DMFC has the same structure as a PEMFC, it uses methanol in a liquid state instead of hydrogen as a reaction gas, which is supplied to anode to produce protons, electrons, and carbon dioxide through an oxidation reaction by aid of a catalyst. The DMFC has inferior cell efficiency to the PEMFC, but since the fuel is injected in a liquid state, the DMFC can be more easily applied to portable electronic devices than the PEMFC.
To improve the power density and voltage of a fuel cell, studies of electrodes, fuels, and electrolyte membranes are being actively conducted. In particular, an attempt has been made to improve the activity of a catalyst used in the electrode. A catalyst used in the PEMFC or the DMFC is generally Pt or an alloy of Pt and another metal. To ensure a competitive price, it is necessary to reduce an amount of the metallic catalyst used. Thus, to reduce the amount of the catalyst while retaining or improving performance of a fuel cell, an electrically conductive carbon material with broad specific surface area has been used as a support and Pt has been dispersed as fine particles in the support, thereby increasing the electrochemically active surface area of the catalytic metal particles.
As the electrochemically active surface area of a catalyst increases, the activity of the catalyst is improved. To improve the electrochemically active surface area of the catalyst, an amount of a supported catalyst used in the electrode can be increased. In so doing, however, the amount of carbon support used is increased and thus the thickness of the electrodes is increased. As a result, the inner resistance of the electrode increases making it difficult to form an electrode. Accordingly, it is necessary to increase the loading of the supported catalyst while retaining the amount of the support used. However, a high degree of dispersion may be obtained by preparing very fine catalytic metal particles when preparing a high loading supported catalyst. In the case of a conventional supported Pt catalyst, the loading of supported Pt is 20–40% by weight. According to Antolini et al., 78 MATERIALS CHEMISTRY AND PHYSICS 563 (2003), in the case of a commercial catalyst available from E-TEK, if the loading of a Pt metal particle in a catalyst is increased from 20% by weight to 60% by weight, the size of the Pt particles increases about 4 times. Thus, although such a catalyst is used in a fuel cell, the benefit of increasing the loading of the supported catalytic metal is not obtained.
U.S. Pat. No. 5,068,161 discloses a solvent reduction method in which H2PtCl6 as a catalytic metal precursor is dissolved in an excessive amount of water as a solvent, and reduced using formaldehyde as a reducing agent. Then, the solution is filtered to remove the solvent and dried in a vacuum to prepare a supported Pt alloy catalyst. However, the size of the catalytic metal particles varies according to the reducing agent and when the concentration of the is catalytic metal is greater than 30% by weight; the catalytic metal particles become excessively large.
In contrast, a method of preparing a carbon supported catalyst in which a catalytic metal precursor is dissolved in an excessive amount of solvent, where the solution is impregnated into a carbon support and subsequently dried to remove the solvent is disclosed in Wendt, 43 ELECTROCHIM. ACTA, 3637 (1998). In so doing, however, a concentration gradient is generated upon drying, thereby causing a capillary phenomenon. Thus, the capillary phenomenon results in an accumulation of the metal salt onto the pore surface of the carbon support. Also, as the loading of the catalyst increases, the size of the catalytic metal particle also increases.